The present inventon relates to a method for manufacturing a hybrid integrated circuit device.
As the base plate for use to the hybrid integrated circuit device, is conventionally well-known the one as shown in FIG. 1. The conventional base plate is manufactured by printing a desirably patterned paste of Ag-Pb system on a ceramic substrate 1, sintering it at a high temperature of 700.degree. C.-800.degree. C. to form a conductive layer 2, and further printing and sintering, if necessary, a paste of RuO.sub.2 (ruthenium oxide) system at desired areas thereon to form a resistive layer 3.
In the case where a power element 4 having large power loss is mounted, as shown in FIG. 2, on the base plate thus formed, a large amount of heat is generated. The entire device must be therefore designed to have an excellent heat radiating characteristic to prevent the device from being damaged by the heat generated. In this case, therefore, a heat radiating metal plate 5 is attached to the back side of ceramic substrate 1 using an adhesive layer 6 such as solder and resin, as shown in FIG. 2. In the case of a hybrid integrated circuit device having such arrangement as shown in FIG. 2, heat generated by the power element 4 cannot be radiated enough by attaching a metal plate to the backside of the substrate. In addition, the conductive layer 2 must be made of noble metal such as Ag-Pb and a different ceramic substrate 1 and heat radiating metal plate 5 must be prepared, thus making the cost of the device high.
Another base plate as shown in FIG. 3 which has the ceramic substrate and heat radiating metal plate formed integral to each other is also provided conventionally. Namely, this base plate is manufactured by flame-spraying a ceramic layer 12 on a heat radiating metal plate 11 and forming a desirably patterned conductive layer 13 on the flame-sprayed ceramic layer 12. In the case of forming the ceramic layer 12 by flame-spraying as described above, the ceramic layer 12 can be formed extremely thin to have a thickness of 100-500.mu., thus enabling excellent heat radiating effect to be attained.
Providing that the quantity of heat conducted through a material is represented by Q, the quantity Q of heat is generally expressed as follows: ##EQU1## wherein Q represents the quantity of heat conducted, K a heat conductivity, A an area where heat is conducted, .DELTA.X the thickness of material, and .DELTA.T a temperature difference between both ends of thickness .DELTA.X. When equation (1) is modified, ##EQU2## when ##EQU3## and this Rth is regarded as heat resistance, it will be understood that the smaller this heat resistance Rth is, the better the heat conductivity becomes.
Heat resistance Rth can be calculated as follows in both cases where the base plate shown in FIG. 2 has the ceramic substrate 1 made of Al.sub.2 O.sub.3 with a thickness of 0.63 mm and the adhesive layer 6 made of epoxy resin with a thickness of 0.020 mm; and where the base plate shown in FIG. 3 has the ceramic layer 12 made of Al.sub.2 O.sub.3 with a thickness of 0.1 mm, providing that the heat conductivity of Al.sub.2 O.sub.3 by which the ceramic substrate 1 of base plate shown in FIG. 2 is formed is 0.260 W/cm.degree. C., that the heat conductivity of flame-sprayed Al.sub.2 O.sub.3 by which the ceramic layer 12 of base plate shown in FIG. 3 is formed is 0.026 W/cm.degree. C., and that the heat conductivity of epoxy resin 6 in the base plate of FIG. 2 is 0.0035 W/cm.degree. C. Heat resistance Rth due to the ceramic substrate 1 and adhesive layer 6 in the base plate shown in FIG. 2 under these conditions is: ##EQU4## Heat resistance Rth of ceramic layer 12 in the base plate shown in FIG. 3 is: ##EQU5## Therefore, the ratio of heat resistance of base plate having such arrangement as shown in FIG. 3 relative to that of base plate having such arrangement as shown in FIG. 2 is: ##EQU6## Namely, the heat resistance of base plate shown in FIG. 3 is less than half the heat resistance of base plate shown in FIG. 2. In other words, the base plate shown in FIG. 3 is more than two times better in heat radiating capacity than the base plate shown in FIG. 2.
A conductive layer 13 shown in FIG. 4 is formed on the ceramic layer 12 of base plate excellent in heat radiating characteristic and shown in FIG. 3. The methods of forming the conductive layer 13 include (1) printing Cu paste of resin base, (2) electroless-plating Cu, Ni and so on and (3) flame-spraying Cu. Method (1) enables sintering to be achieved in atmosphere and at a relatively low temperature (120.degree. C.-150.degree. C.). The conductive layer thus formed according to method (1) is therefore inexpensive, but weak relative to mechanical impact. Method (2) makes the cost high and is unpractical. Method (3) can be relatively easily carried out similarly to the spraying of ceramic and is inexpensive. It is therefore most desirable to form the conductive layer 13 according to method (3) of flame-spraying Cu.
Conventional method (3), however, causes a problem on the strength with which the flame-sprayed Cu layer 13 adheres to the ceramic layer 12. Because the ceramic layer 12 is formed by flame-spraying, through pores are present among ceramic material particles which form the ceramic layer 12, and cause the insulating resistance or the dielectric strength of ceramic layer 12 to be lowered. Even if Cu layer 13 is flame-sprayed on the ceramic layer 12 to avoid it, the insulating resistance thereof becomes extremely small when used for a long time under severe conditions or when left for a long time in an atmosphere at a temperature of 60.degree. C. and a humidity of 90%, for example. In order to solve this problem, it is necessary to fill through pores in the ceramic layer 12. The ceramic layer 12 is therefore conventionally immersed with a thermosetting resin to fill through pores therein. Thermosetting resins employed to fill through pores are required to have such characteristics that their viscosity is low and their processability is excellent; that no solvent is needed to lower their viscosity; and that their thermosetting temperature is low and their thermosetting time is short. Resins to meet these requirements are thermosetting resins including epoxy resin, thermosetting acrylic resin, thermosetting polyurethane resin, diaryl phthalate resin, thermosetting polybutadiene resin, silicone resin, thermosetting polyimide resin, bismaleimide resin and so on. Some of these resins may be mixed with one another or one of these resins may be blended with a multifunctionality monomer of low molecular weight.
When the conductive layer 13 is formed by flame-sprayed Cu on the ceramic layer 12 to which the through pores filling process has been added using one of these resins, however, the strength with which the conductive layer 13 adheres to the ceramic layer 12 becomes extremely low. It is because the surface of ceramic layer 12 to which the through pores filling process has been added becomes so smooth with the resin immersed as to cause difficulty in depositing the flame-sprayed copper on the surface of ceramic layer 12. The conventional method to solve this problem comprises forming the conductive layer 13 by flame-spraying Cu all over the ceramic layer 12 also formed by flame-spray, immersing the ceramic layer 12 and conductive layer 13 with thermosetting resin to fill through pores therein, polishing the surface of conductive layer 13, selectively printing etching resist on the conductive layer 13, and selectively etching the conductive layer 13. According to this method, however, etching time is made long because of conductive layer 13 immersed with thermosetting resin and takes substantially five times as compared with that in the case where no thermosetting resin is contained.